nvk: Enable EXT_nested_command_buffer.

This implements VK_EXT_nested_command_buffer. Closes #10688 (closed).

vkCmdExecuteCommands is just a matter of copying pushes across and invalidating some state. On top of that, the only mentions of "PRIMARY" or "SECONDARY" in code are in nvk_cmd_buffer_begin_compute and nvk_cmd_buffer_begin_graphics and both of those don't seem like they are impacted by nesting. Additionally, I can't see a good reason why any of the features of the extension would be unsupported. The proprietary driver supports all features as well.

As far as I can tell there are no CTS tests for this extension unfortunately. All supported tests pass.